Field of the Invention
The invention relates to a multilayer protective coating for protecting a component from corrosion and oxidation at a high temperature and from excessive thermal stress, a process for coating a component with a multilayer protective coating, and a component, in particular a gas turbine component, which is coated with a multilayer protective coating.
A metal article, in particular a gas turbine blade, which is known from Published European Patent Application 0 266 299 A2, corresponding to U.S. Pat. No. 4,861,618, has a protective coating for protecting the metal article at elevated temperature. The protective coating includes a ceramic heat insulation layer of zirconium oxide with yttrium oxide added, and an adhesion layer, connecting the heat insulation layer to the metal article, including an alloy composed of the following elements by weight: 15-40% cobalt, 10-40% chromium, 6-15% aluminum, 0-2% hafnium, 0-7% silicon, 0.01 to 1.0% yttrium and the remainder nickel. The adhesion layer and the heat insulation layer can be applied to the metal article by atmospheric plasma spraying.
Metallic protective coatings for metallic components, in particular for gas turbine components, which are intended to increase their corrosion resistance and/or oxidation resistance, are known in the prior art. In the case of stationary gas turbines with material temperatures of about 950.degree. C. and gas turbines in aircraft engines with inlet temperatures of about 1100.degree. C., an inlet temperature increase which influences the thermodynamic efficiency has been achieved by the use of specially developed alloys as base materials for components, such as guide vanes and rotor blades, that are subjected to high thermal loads. It has been possible to consider temperatures substantially above 1000.degree. C. for those components, particularly as a result of the use of monocrystalline superalloys. In addition to thermomechanical stresses, such a component is also exposed to chemical attack, for example by a combustion gas with a temperature up to more than 1300.degree. C. In order to ensure sufficient resistance to such an attack, the component is usually covered with a metallic protective coating. The protective coating must have sufficiently good mechanical properties. The protective coating should be sufficiently ductile to be able to comply with any deformations of the base material, particularly with regard to the mechanical interaction between the protective coating and the base material of the component. It should also have as little susceptibility to cracking as possible, in order to prevent corrosion and oxidation of the base material.
A plurality of protective coatings is known under the general term MCrAlY, where M represents at least one of the elements from the group including iron, cobalt and nickel and further essential components are chromium, aluminum and yttrium or a metal which is selected from the group including scandium and the rare earth elements and is equivalent to yttrium.
Such an alloy which is used in a process for improving the oxidation resistance of a superalloy component covered with a protective coating is described in U.S. Pat. No. 4,451,299. The protective coating contains 15-45% chromium, 7-20% aluminum and 0.1-5% yttrium (data in each case in percent by weight). The yttrium may be replaced by lanthanum and cerium. In addition, the protective coating optionally contains a mixture of further elements from the group including platinum, rhenium, silicon, tantalum and magnesium in an amount of up to 10%. That U.S. patent does not reveal the extent to which the addition of one of those optional elements contributes to the improvement in the oxidation resistance of the superalloy. Furthermore, the wide ranges for possible admixing which are specified in little detail provide no qualification of the protective coating for particular conditions, for example in a stationary gas turbine with high inlet temperature, if it is operated not only under full load but also under part load over prolonged periods.
A protective coating which is intended to improve the corrosion properties and oxidation properties in a surface temperature range of 600.degree. C. to 1150.degree. C. is described in Published European Patent Application 0 412 397 A1, corresponding to U.S. Pat. No. 5,268,238. The protective coating contains an amount of 1 to 20% rhenium in addition to 22-50% chromium, 0-15% aluminum and 0.3-2% yttrium or another element from the group including the rare earths. The action of the rhenium with regard to an improvement in corrosive and oxidizing effects resembles the positive actions of platinum. Due to the good thermal conductivity of the metallic protective coating, a component covered with the protective coating is exposed to virtually the same thermal load as the protective coating itself.
Published International Patent Application WO 89/07159 describes a two-layer metallic protective coating including two different alloys. An outer alloy of those alloys is covered by the general term MCrAlY and contains (stated in % by weight) 15 to 40% chromium, 3 to 15% aluminum and 0.2 to 3% of at least one element from the group including yttrium, tantalum, hafnium, scandium, zirconium, niobium, rhenium and silicon. That alloy in turn is preferably surrounded by a thermal barrier layer, in particular on a component cooled from the inside, for protection against particularly high temperatures. The thermal barrier layer may be zirconium oxide with added yttrium oxide. Oxidation of the alloy prior to application of the thermal barrier layer is envisaged in order to prevent possible flaking of the thermal barrier layer from the alloy.
Published European Patent Application 0 532 150 A1 describes a component covered with a protective layer and including a superalloy, for example a turbine blade. The protective coating contains at least 2% (stated in % by weight) tantalum as an essential element in addition to chromium and aluminum. The protective coating optionally contains up to 1% yttrium and up to 4% rhenium. In the case of a protective coating including such an alloy, Published European Patent Application 0 532 150 A1 considers a coating including a ceramic thermal barrier to be possible but does not discuss the interaction between the alloy and the thermal barrier, which interaction is critical in the case of temperature changes.
U.S. Pat. Nos. 4,055,705, 4,321,310 and 4,321,311 relate to protective coatings for gas turbine components including superalloys based on nickel or cobalt. According to those patents, a protective coating includes a ceramic heat insulation layer which preferably has a columnar crystal structure and is present on an adhesion promoting layer which in turn is present on the base material of the gas turbine component and binds the heat insulation layer to the base material. The adhesion promoting layer includes an alloy of the type MCrAlY. It is essential that the adhesion-promoting layer develops, between itself and the heat insulation layer, a thin aluminum oxide layer on which the heat insulation layer is anchored.
U.S. Pat. No. 5,087,477 presents a process for applying a ceramic heat insulation layer to a gas turbine component. That process includes a physical vapor deposition (PVD) process, in which compounds that are intended to form the heat insulation layer are vaporized through the use of an electron beam and an atmosphere having a defined and carefully controlled oxygen content is produced in the environment of the component.